Ambient noise tomography of the Ordos Block and the NE Tibetan Plateau

The NE Tibetan Plateau (TP) and the Ordos Block (OB) region covers a transition from the high plateau to the stable block. The tectonic regime changes from compression of the NE TP to extension of the SE margin of OB. To understand the dynamics of the interaction between plateau growth and cratonic evolution, we perform ambient noise tomography of the region. We use continuous seismic data from permanent national and provincial stations as well as temporary broadband stations we deployed. We obtain Green functions of both Rayleigh and Love waves from cross-correlation of vertical and traverse components, respectively. Both group and phase velocities are measured. We derive tomographic dispersion maps from 8 to 40 s from the dispersion measurements, which can robustly resolve the relatively fine structural elements such as the Weihe-Shanxi rift, the Taihang mountain, and the Lvliang mountain. We also invert 3-D shear-wave (SV and SH) velocity structure of the crust and uppermost mantle from the Rayleigh wave and Love wave dispersion maps. At shallow depths, the Ordos Block and the Weihe rift show pronounced slow velocity anomalies, whereas the mountain ranges are imaged as fast. At middle-lower crust, the NE Tibet is characterized by obvious low velocities which, however, does not extend eastward to the Qinling orogeny (lying between the Ordos and Sichuan Block). Slow anomalies appear beneath the Weihe-Shanxi rift from the surface to mid-lower crust, suggesting the rift extends through the crust. In the NE Tibet, the radial anisotropy changes sign, from vertically oriented fabric in the upper crust to horizontally oriented fabric in mid-lower crust, indicating that the deformation style varies with depth.